I don't think anyone here was suggesting "making superficial observations". I have a fourteen year old, and on any given day there are at least five to ten kids from 13 to 16 hanging out around my house. I watch them, I hear them, and most importantly, I TALK to them.
I completely agree that trying to use up to date "cool words" and referring to fashion trends will make the work seem dated within a matter of months -- teens change their idea of what's hot and what's not as often as the rest of us change our socks. On the other hand, there are some things that are consistetly important to teenagers -- relationships, school, dealing with parents, feeling like you fit in, trying to cope with sexuality, etc.
The fact is, you can't write realistically about teenagers unless you have observed them in their natural environment. Hence my earlier "gorilla" statement
Also, if you're reading YA and teen novels to get an idea of what is popular, don't just grab a book off the shelf. Ask a teen what they're reading, or ask the librarian what the kids are checking out. My library has a section for Teen Fiction, and it includes copies of Michael Crighton novels and Jane Austen -- the librarian told me she could count on one hand the number of kids who had taken out Jurassic Park.
Another important thing to remember is that teens are way smarter and wolrld-savvy than we give them credit for. They're certainly more open about a lot of things than we were as kids. I'm 37, and I know there are things that my teenage friends and I would NEVER have discussed with other people that kids today are quite open and willing to discuss. Kids are more open about sexuality now than we were twenty years ago, just for starters. They're more also politically aware, they have healthy amount of activism in them when something matters, and they're a lot more independent and self-sufficient at younger ages.
Just like with any demographic, if you're writing for them, you better know your audience.